The Guarani Aquifer is a geological fact of almost absurd scale - a sandstone formation holding water beneath Brazil, Paraguay, Uruguay, and Argentina, sealed under basalt that erupted when the Atlantic was opening. Depth does the heating. Water drawn from far enough down arrives at the surface already warm, which is why the pools at Thermas dos Laranjais run somewhere between the mid-twenties and high thirties in Celsius without a boiler anywhere in the plant. That is the park's founding advantage and it is not one a competitor can buy. Warm water is expensive to make and free to find, if you happen to be standing on the right rock.

Construction began in the mid-1980s, driven by the businessman Benito Benatti, with the architect Jorge Noronha designing the grounds. The park opened on November 7, 1987, in a country that had very little in the way of a water-park industry and a region with no tourism to speak of. The name is unglamorous and honest - laranjais means orange groves, which is what covered the land. Four decades on, it occupies more than 300,000 square metres, holds over sixty attractions, and can take up to twenty thousand people through the gates in a single day.

The inventory reads like a catalogue of everything the form has invented: complexes of water slides stacked into towers, a surf track, a lazy river that loops the grounds, relaxation pools where the water is warmest and the average age climbs sharply, a dedicated children's area, and two artificial beaches with sand and a horizon line, several hundred kilometres from the actual coast. That last detail explains a great deal about the park's appeal. Interior São Paulo is hot, prosperous, densely populated, and nowhere near the ocean. Thermas dos Laranjais sells the beach to a region that does not have one, and it sells it heated.

The attendance figures are the part that startles people. The annual index published by the Themed Entertainment Association with its research partners has repeatedly ranked Thermas dos Laranjais the most visited water park in Latin America, and in 2023 it was the most visited in the Americas and second in the world - ahead of every park in Orlando, behind only Chimelong Water Park in Guangzhou - with close to two million visitors for the year. Rankings shift as parks open and travel patterns move, and the park has traded places in the global top five since. Reputation has followed the numbers: TripAdvisor users ranked it the eleventh best water park in the world in 2015, and fifth by 2017.

Olímpia began as Fazenda Olhos d'Água, a nineteenth-century coffee estate named for the springs on the property - the water was there in the name from the start. The settlement became a district of Barretos in 1906 and took the name Vila Olímpia in honour of Maria Olímpia Rodrigues Vieira, a founder's daughter. It was a town of around 55,000 people at the 2020 census, and thermal tourism now generates on the order of 350 million reais a year in the municipality. The São Paulo State Legislative Assembly formally designated Olímpia a tourist municipality in 2014. The park itself belongs to the World Waterpark Association and was the first water park in Brazil to sign the United Nations Global Compact, committing to its principles and the Sustainable Development Goals. Hotels line the approach roads now, and the old railway station and the first hospital survive as listed landmarks - relics of the coffee town that was here before anyone drilled a hole in the ground and found something better than oil.